Store custom field default values as JSON

[jeremystretch]

Environment

• Python version: 3.6.9
• NetBox version: 2.10-beta1

Proposed Functionality

The CustomField model's default field should be a JSONField rather than a CharField.

Use Case

Using a JSONField permits the storage of a default value in its native type, and avoids the need for clunky deserialization (e.g. storing the string "false" for a boolean field).

Database Changes

Migrate the default field to a JSONField and update all existing values accordingly.

External Dependencies

None